The reversibility of the spontaneous polarization makes ferroelectric materials promising candidates for use as storage media in future non-volatile memory devices. Binary information is stored in the two remanent polarization states by applying an appropriate switching voltage to a ferroelectric capacitor. After poling the capacitor into the desired state, the polarization is preserved without the application of an external field.
Ferroelectric materials can form the basis for data storage devices, where digital “1” and “0” levels are represented by the electric polarization of a ferroelectric film pointing “up” or “down”. Storage devices based on a ferroelectric storage medium include Ferroelectric Random Access Memory (FeRAM) and scanning-probe storage systems (“FE-probe”).
In a FeRAM memory cell the storage element includes a thin ferroelectric film sandwiched between fixed, conductive electrodes. To write a bit to such a cell, a voltage pulse of either positive or negative polarity is applied between the electrodes in order to switch the internal polarization of the ferroelectric film to the “up” or “down” state, respectively. To read back the data from the FeRAM cell, a read voltage of a certain polarity (e.g. positive) is applied, which switches the polarization of the ferroelectric film in cells storing a “0” (“down” polarization), while having no effect in cells storing a “1”. A sense amplifier measures the charge flow that results when the polarization switches, so that a current pulse is observed for cells which stored a “0”, but not for cells which stored a “1”, thus providing a destructive readback capability.
Probe storage devices have been proposed to provide small size, high capacity, low cost data storage devices. A probe storage device based on ferroelectric thin films uses one or more small, electrically conducting tips as movable top electrodes to store binary information in spatially localized domains. Binary “1's” and “0's” are stored in the media by causing the polarization of the ferroelectric film to point “up” or “down” in a spatially small region (domain) local to the electrode, by applying suitable voltages to the electrode. Data can then be read out by a variety of means, including sensing of piezoelectric surface displacement, measurement of local conductivity changes, or by sensing current flow during polarization reversal (destructive readout).
In ferroelectric probe storage, a conducting probe tip scans on the surface of a ferroelectric media to provide an electric field on the media for write and/or read. The track width of a probe storage is nominally defined by the dimension of the probe tip perpendicular to the scan direction, while the bit length by the distance of the probe tip travels along the scan direction with a certain applied electric voltage; both of them are based on an assumption that the electric field from the probe tip only affects the ferroelectric media underneath the tip footprint. Actual media area affected by an electrically biased probe tip, however, is larger than the tip footprint due to fringing electric field from the tip, which is called electric field blooming effect. Ferroelectric imprint causes the blooming on up and down bits to be an asymmetric size. In other words, the written bit size with a certain polarization state is different from that with opposite state. As a result, a polarization-dependent variation of track width and bit length is induced, which is undesirable for a ferroelectric probe storage device. A non-uniform track width would lead to erasure of neighboring tracks and would cause strong variations in the signal magnitude for opposite bit states (“1” and “0”), which would complicate the data analysis and reduce the ultimate storage density. A non-uniform bit length will cause an additional jitter, an increase of the bit error rate and an overall reduced writability/readability especially for high areal density.
Therefore, therefore the compensation of the asymmetric blooming effect on bit size with opposite polarization directions caused by ferroelectric imprint in ferroelectric probe storage. There is also a need to improve writability and write voltage efficiency in other thin-ferroelectric film based memory systems.